The long-term goal of this project is to understand better the genetic basis of skeletal fragility. In particular, these studies aim to determine i) how multiple genetic loci interact to regulate bone morphology and quality, and ii) how those critical intermediary traits determine the whole bone mechanical properties that define the tissue's susceptibility to fracture. The goal of this proposal is to determine how genetic background regulates the intermediary bone traits (morphology, quality) that contribute to whole bone mechanical function. The principal outcome is the identities of indices of cellular activity (i.e. patterns of matrix formation, resorption, mineralization) occurring during development that give rise to genetic variation in mechanically relevant bone traits. These biological processes precede temporally and are, thus, expected to be deterministic of adult bone shape and quality. The working hypothesis is that traits specifying bone morphology and quality are interdependent and coordinately regulated. Because complex intermediary traits result from the actions of multiple genes, Recombinant Inbred (RI) mouse strains derived from AJ and B6 mice will be utilized to facilitate analysis. Networks of functional interactions among intermediary skeletal traits will be established by measuring the tendency for traits to cosegregate after genetic randomization of the parental genomes in the RI strain panel. Using this novel systems approach, networks describing functional interactions between cellular indices and properties related to fragility will be constructed in relation to structural hierarchy and age, first for females (Aim 1) and then for males (Aim 2). We will then test whether the identified mechanistic controls of hierarchical traits are shared across genotypes (Aim 3). Successful completion of these studies will identify determinant cellular indices that can be used in future studies as phenotypic markers to identify relevant genetic loci. These markers offer the advantage that genetic loci will be closely associated with critical biological processes. Further, the network analysis will reveal how these genes are ultimately related to complex whole bone properties related to fragility. These functional interactions are critically important for developing a comprehensive systems analysis of bone that will generate genetic-based strategies for early diagnosis and prophylactic treatment of osteoporosis.